The present invention relates to a combustion device of the type in which liquid fuel is atomized for combustion by an ultrasonic atomizer.
The conventional combustion devices of the type for atomizing liquid fuel for combustion may be further divided into the following types;
(I) the combustion device incorporating the ultrasonic atomizer,
(II) a gun type combustion device in which fuel is pressurized to the order of 7 kg/cm.sup.2 by a gear pump or the like and then is atomized when injected through a nozzle, and
(III) a rotary type combustion device in which liquid fuel is atomized by the centrifugal force produced by the rotation of a rotary disk or the like. The atomized fuel particles are mixed with the combustion air and burned. However, almost all of the conventional combustion devices of the type for atomizing the liquid fuel are not adapted for the low-rate combustion, for instance, the combustion at a rate less than 10,000 KCal/Hr. The reason is that the flame "blow-off" tends to occur so that the flame cannot be maintained in the stablized state, thus resulting in the incomplete combustion. In case of the gun type combustion device, the lower the combustion rate, the smaller the diameter of the nozzle hole must be made. However, in practice it is extremely difficult to drill such small diameter hole in the nozzle, and the small diameter nozzle hole tends to be clogged with foreign matter. Therefore, the conventional combustion devices of the type described can sustain the combustion only at a predetermined combustion rate. Furthermore the operation thereof for ignition, burning and extinguishing is controlled by the on-off control system. When the conventional combustion device is incorporated into a boiler for furnace in order to supply hot water or hot air of a predetermined temperature, the combustion device must be turned on and off at a short cycle so that the following problems arise:
(I) SINCE THE COMBUSTION DEVICE, THE HEAT EXHANGER, THE COMBUSTION CHAMBER AND OTHER ASSOCIATED EQUIPMENT OR DEVICES ARE REPEATEDLY HEATED AND COOLED, THEY ARE SUBJECTED TO THERMAL DISTORTIONS AND DEFORMATIONS, RESULTING IN A SHORT SERVICE LIFE;
(II) SINCE THE COMBUSTION DEVICE AND ITS ASSOCIATED CONTROL UNIT ARE TURNED ON AND OFF VERY FREQUENTLY, THEIR SERVICE LIFE IS REDUCED, AND
(III) SINCE THE TEMPERATURE OF THE COMBUSTION CHAMBER IS LOW IMMEDIATELY AFTER THE IGNITION, THE COMPLETE COMBUSTION CANNOT BE EXPECTED AT THE INITIAL STAGE. Therefore, when the turn-on and -off operations are cycled at a short cycle, the number of instances of incomplete combustion is increased, resulting in the increase in emission of smoke, soot and other pollutants such as carbon monoxide.
In the combustion device of the type including the ultrasonic atomizer, liquid fuel forms a very thin film upon the atomizing surface by the surface tension of the liquid fuel, and when the atomizing surface oscillates at an ultrasonic frequency, the thin film is broken so that the liquid fuel may be atomized into very finely divided particles. However, the kinetic energy of the atomized fuel particles is considerably lower than that of the particles atomized by other types of atomizers such as a pressure type atomizer. As a result, the atomized fuel particles cannot form the desired spray pattern and tend to descend under the influence of gravity immediately after they have left the atomizing surface. As a result, the density of the atomized fuel particles is considerably high in the vicinity of the atomizing surface. That is, the atomized fuel particles are not satisfactorily mixed with the combustion air in the ignition zone in a suitable ratio so that the ignitionability is adversely affected. Consequently, the pulsation and leap of flame occur, causing the combustion noise and adversely affecting the combustion. Since the atomized fuel particles have only considerably low kinetic energy, they are easily carried out by the combustion air with the result in the increase in unburned fuel.
In general, the nozzle, which is a part of the ultrasonic atomizer, is not adapted to satisfactorily atomize the liquid fuel regardless of its viscosity, surface tension and other properties. In order to atomize a large quantity of liquid fuel, the input must be increased because the thickness of liquid fuel film on the atomizing surface is increased. If the excessive input is applied, the thermal stresses are produced, causing the short service life.
The conventional combustion device incorporating the ultrasonic atomizer has a further defect that the mechanical oscillations of the ultrasonic transducer are transmitted to other parts, thus increasing the noise.
In order to overcome the above and other problems encountered in the prior art combustion devices incorporating the ultrasonic atomizer, there have been devised and demonstrated various improvements of the combustion device. Among these may be cited U.S. Pat. No. 3,275,059, Nozzle System and Fuel Oil Burner, granted to J. E. McCullough. However, the combustion rate of this fuel oil burner is constant. That is, the fuel oil burner cannot be selectively switched between the high and low combustion rates. In other words, it cannot control the thermal energy liberated by the combustion depending upon a demand. It has a further defect that the uniform supply of air cannot be ensured.